The present invention relates to a sheet feeder device provided in an image forming apparatus such as a printer, facsimile apparatus or copying machine for feeding such sheets as document sheets or recording sheets to the image forming apparatus. The present invention also relates to an image forming apparatus provided with such a sheet feeder device.
In an image forming apparatus such as a copying machine or a printer, a given sheet feeder device feeds such sheets as document sheets or recording sheets from a sheet tray such as a document tray or a paper feed tray toward the image forming section.
Such a sheet feeder device picks up sheets one by one by rotating a given pick-up roller pressed against the uppermost surface of a stack of sheets placed on a sheet tray. In order to prevent plural sheets from being fed at a time the sheet feeder device passes sheets thus picked up between a given pair of separating roller and separating pad (friction pad) to feed the sheets one by one separately. In feeding a series of sheets successively, it is required that the feeding of a succeeding sheet fail to start during the feeding of the preceding sheet. For this purpose, the pick-up roller of the sheet feeder device is supported on a pivotable arm portion so as to be capable of pivoting toward and away from the sheet stack. The pick-up roller is positioned so as to be capable of separating away from the sheet stack to assume a predetermined retracted position after the separating roller has taken over the transport of one sheet. By thus causing the pick-up roller to reciprocate between the sheet feed position and the retracted position, sheets can be fed as spaced one from another.
With the increase in the sheet load that can be carried by a sheet tray (sheet carrying section) of an image forming apparatus in recent years, the spacing between adjacent sheets under feeding exerts increasing influence upon the processing speed of the image forming apparatus.
Under such circumstances, Japanese Patent Laid-Open Publication No. HEI 02-231320, for example, has proposed the art of reducing the spacing between adjacent document sheets by making higher the sheet transport speed for a feed path up to an idle roller which temporarily stops each document sheet to register the leading position thereof than the document reading speed.
However, the prior art described in this patent publication involves a problem that the increased document sheet transport speed is likely to cause document sheets to be fed askew or to make thin document sheets become wrinkled. In addition, when the feed path is curved, the increased transport speed is likely to cause a paper jam to occur.
If the time required for the pick-up roller to move between the retracted position and the sheet feed position in which the pick-up roller abuts against a stack of sheets to feed each sheet is shortened, the spacing between adjacent sheets can be reduced without increasing the sheet transport speed and, hence, the above-described problem will not occur.